1. Field of the Invention
The present invention relates to a waveform dispersive compensation method having a function, such as pulse waveform shaping, and, more particularly, to an adaptive dispersion compensating element used in ultrahigh-speed optical fiber communication.
2. Description of Prior Art
Recently, in optical fiber communication, its introduction into an optical access system is advancing, to say nothing of a trunk line system. In many 1.3 micron band zero dispersion fibers laid at present, when transmission is performed using light having a wavelength in a 1.5 micron band, a wavelength dispersion of about 17 ps/kmxe2x88x92nm can be found in an optical fiber. Accordingly, when the transmission distance is made longer or when the transmission rate becomes fast, a means for controlling dispersion becomes necessary to prevent deterioration of an optical signal.
A typical means that controls conventional dispersion is a dispersion compensator that uses a chirp Bragg grating having the structure in which a cycle of refractive index modulation is continuously changed. A fiber Bragg grating that forms a diffraction grating in a core of this optical fiber becomes an element whose position of reflection depends on an optical wavelength by forming a diffraction grating (chirp Bragg grating) that has a characteristic of reflecting light of a specific wavelength and continuously changes a pitch toward the major axis direction of an optical fiber. A dispersion compensator can be constituted using this feature. This chirp Bragg fiber grating becomes compact and has the same function as a dispersion compensating fiber by combining with an optical circulator.
However, in most chirp Bragg gratings, dispersion and reflection characteristics were static. Desirably, they should have a diffraction grating that can change a band or dispersion with satisfactory control against many applications, such as dispersive compensation.
One of the attempts that introduces a dynamically adjustable chirp into a chirp Bragg fiber grating as a conventional example can be found in an xe2x80x9cOptical Diffraction Device Having an Adjustable Chirpxe2x80x9d disclosed in Japanese Unexamined Patent Publication No. 2000-137197.
FIG. 1 shows a process useful for providing an example in which a chirp diffraction grating is adjusted using a block diagram. In FIG. 1, Process A is a xe2x80x9cPreparation of a waveguide including a diffraction gratingxe2x80x9d, Process B is xe2x80x9cCoating of a diffraction grating area using a variable-resistance thin filmxe2x80x9d, or Process C is xe2x80x9cPackaging of a devicexe2x80x9d.
In FIG. 1, the operation is described below. As shown in Process A, the first process is to prepare an optical waveguide of a fixed length including an optical diffraction grating. Desirably, a waveguide should be an uncoated fiber, but can include an electrically insulated resistor thin film of uniform resistance. The waveguide ought to be either single mode or multi mode. The diffraction grating ought to be either a Bragg diffraction grating or a long cycle diffraction grating. The next process, as shown in Process B, is to coat a waveguide with a thin film of a resistance material in which local resistance increases substantially in succession along the length of a diffraction grating. The third process (Process C) (this is performed as occasion demands) is to package a device for operation.
FIG. 2 shows a schematic sectional view of a waveguide diffraction grating device having an adjustable chirp as a specific configuration example. In FIG. 2, number 10 is an optical fiber, 11 is a diffraction grating, 12 is refractive index perturbation, 13 is a substrate, and 14, 15 are electrodes.
An optical waveguide diffraction grating having an adjustable chirp includes a waveguide diffraction grating that thermally contacts an electrically controllable thermal conversion substrate whose temperature changes along the length of a diffraction grating. Because a thermal conversion substrate generates a temperature gradient along a diffraction grating, it generates heat on a fiber or can remove the heat from the fiber. As an example, the thermal conversion substrate is a resistive coat in which local resistance changes along the length of the diffraction grating. A current that passes through a thin film generates a temperature gradient along a diffraction grating that is almost proportional to the local resistance of the thin film and the size of a chirp can be adjusted by the current. A device that is obtained is simple and compact, and the power is efficient.
However, in a means that uses the chirp diffraction grating, it is unknown how chirp characteristic control for compensating dispersion is performed in accordance with a change of the transmission state and a change of the transmission distance. Accordingly, the means had a problem that cannot flexibly be solved in accordance with the optical pulse transmission of practical optical communication. Further, a resistor element that generates a temperature gradient has the configuration in which heat output is controlled by changing a value of resistance in accordance with a change in the local thickness of a thin film. However, the means had a problem that it is difficult to control higher order diffusion (exceeding tertiary diffusion) than wavelength diffusion (secondary diffusion) in such configuration.
The present invention has been made in view of solving the above prior art and provides a device that adaptively performs decentralized control in an optical fiber transmission path, such as performing dispersive compensation and waveform shaping in an optical fiber transmission.
To attain this object, according to an aspect of the present invention, the adaptive dispersion compensating element is provided with a chirp Bragg grating formed in an optical fiber, a temperature gradient impressing means that impresses a temperature gradient along the longitudinal direction of the chirp Bragg grating, a spectral resolving means that spectrally resolves the output light from the chirp Bragg grating, a detecting means that detects the output light from the spectral resolving means, and a controlling means that performs feedback control of the temperature gradient impressing means based on the output from the detecting means.
According to another aspect of the present invention, the adaptive dispersion compensating element can provide a compact and high-stability adaptive dispersion compensating element that adaptively performs dispersive compensation monitoring an optical signal in an optical fiber transmission path, such as performing dispersive compensation or waveform shaping in optical fiber transmission in accordance with the above configuration.
According to another aspect of the present invention, the adaptive dispersion compensating element is provided with a chirp Bragg grating formed in an optical fiber, a temperature gradient impressing means that impresses a temperature gradient along the longitudinal direction of the chirp Bragg grating, a spectral resolving means that spectrally resolves the output light from the chirp Bragg grating, a detecting means that detects the output light from the spectral resolving means, and a controlling means that performs feedback control of the temperature gradient applying means based on the output from the detecting means, and has operation that adaptively performs dispersive compensation monitoring an optical signal in an optical fiber transmission path, such as performing dispersive compensation or waveform shaping in optical fiber transmission.
Further, according to another aspect of the present invention, the adaptive dispersion compensating element is an adaptive dispersion compensating element whose temperature gradient is a nonlinear gradient that is impressed to the longitudinal direction of a chirp Bragg grating and has operation that adaptively performs dispersive compensation monitoring an optical signal in an optical fiber transmission path, such as performing dispersive compensation or waveform shaping in optical fiber transmission.
Moreover, according to another aspect of the present invention, the adaptive dispersion compensating element has multiple areas in which a chirp Bragg grating is provided with a nonlinear chirp characteristic and a temperature gradient impressing means independently impresses a temperature gradient to the multiple areas respectively, and has operation that compensates residual dispersion, such as higher order dispersion.
Besides, the same effect can also be obtained in an adaptive dispersion compensating element whose side of an optical fiber in which a chirp Bragg grating is formed is polished.
Further, the same effect can also be obtained in an adaptive dispersion compensating element whose side of an optical fiber in which a chirp Bragg grating is formed has an uneven shape.
Moreover, according to another aspect of the present invention, the adaptive dispersion compensating element has a first chirp Bragg grating formed in an optical fiber, a first temperature gradient impressing means that impresses a temperature gradient along the longitudinal direction of the first chirp Bragg grating, a second chirp Bragg grating formed in the optical fiber that receives the output light from the first chirp Bragg grating, a second temperature gradient impressing means that impresses a temperature gradient along the longitudinal direction of the second chirp Bragg grating, a spectral resolving means that spectrally resolves the output light from the second chirp Bragg grating, a detecting means that detects the output light from the spectral resolving means, and a controlling means that performs feedback control of the first temperature gradient impressing means and the second temperature gradient impressing means based on the output from the detecting means, and the first chirp Bragg grating and the second chirp Bragg grating are adaptive dispersion compensating elements that form a chirp in the reverse direction and have operation that cancels secondary dispersion generated in these dispersion compensating elements and compensates only higher order dispersion, such as tertiary dispersion.
Further, according to another aspect of the present invention, the adaptive dispersion compensating element is an adaptive dispersion compensating element whose temperature gradient impressed along the longitudinal direction of a first chirp Bragg grating and a second chirp Bragg grating is a nonlinear gradient and has operation that adaptively performs dispersive compensation monitoring a signal in an optical fiber transmission path, such as performing dispersive compensation and waveform shaping in optical fiber transmission.
Moreover, according to another aspect of the present invention, the adaptive dispersion compensating element has multiple areas in which a first chirp grating and a second chirp grating have a nonlinear chirp characteristic respectively and a first temperature gradient impressing means and a second temperature gradient impressing means independently impress a temperature gradient to the multiple areas respectively and has operation that compensates residual dispersion, such as higher order dispersion.
Besides, the same effect can also be obtained in an adaptive dispersion compensating element whose side of an optical fiber in which a chirp Bragg is formed is polished.
Further, the same effect can also be obtained in an adaptive dispersion compensating element whose side of an optical fiber in which a chirp Bragg has an uneven shape is polished.
According to another aspect of the present invention, the adaptive dispersion compensating element is an adaptive dispersion compensating element in which a spectral resolving means is provided with a grating coupler having an optical fiber and a diffraction grating formed in the optical fiber and has operation that simply performs the optimum control by easily performing spectral resolution of an ultrahigh speed optical pulse of femto-second levels and performing decentralized control based on the result.
As described above, according to another aspect of the present invention, the adaptive dispersion compensating element has a means that impresses a temperature gradient along the longitudinal direction of a chirp Bragg grating provided in an optical fiber, a means that applies a tensile force to the longitudinal direction of the chirp Bragg grating, a means that detects a signal light passing through the chirp Bragg grating through spectral resolution, and a means that performs feedback control based on spectral components of a detected optical frequency. Consequently, an adaptive controlling element that becomes a compact and high-stability device and performs dispersive compensation in an optical fiber transmission path with adaptability, such as performing dispersive compensation and waveform shaping in optical fiber transmission can be realized.
Such objects and advantages of the present invention will further evident from the following embodiments described with reference to the appended drawings.